Dual post elevators are used in applications where it is not desirable to drill a hole for a hydraulic jack. As opposed to a single post elevator, where the jack is located under the car, in dual post elevators a pair of jacks are located on opposite sides of the car. The inner plunger of the jack is connected to the top of the car, whereas the outer cylinder of the jack is supported by the ground.
For hydraulic jacks having a single extending cylinder, the height that car can be raised is limited essentially to the height of the jack. It is therefore desirable in many dual post applications to employ telescoping jacks, e.g., jacks having an inner plunger coupled to the car, the outer cylinder fixed relative to the ground, and one or more intermediate cylinders.
When dual post telescoping jacks are used in an elevator system, there exists the problem that, over time, one or both of the jacks may get out of synchronization due to loss of fluid in the upper chamber, as described below with reference to FIG. 1.
FIG. 1 illustrates, in somewhat simplified form, a telescoping jack 10. The jack includes a first cylinder 14, which is normally fixed relative to the ground. An intermediate cylinder 16 is disposed within the first cylinder 14, and slidable relative thereto through a hydraulic seal 18, which is secured to the first cylinder 14 by a seal collar, or housing 20. An inner plunger 32 is disposed in the intermediate cylinder 16, and slidable relative thereto through a hydraulic seal 28. The hydraulic seal 28 is secured in an intermediate seal housing 29. As shown, the intermediate seal housing 29 extends outwardly from the central cylinder axis 30 further than the intermediate cylinder 16 itself. The inner plunger 32 is preferably closed off at its lower end by a stop 34.
The intermediate cylinder 16 includes a piston 22 which is slidingly mounted in the first cylinder 14 and includes a hydraulic seal 24 between the piston 22 and the adjacent cylinder wall. The piston 22 divides the main cylinder 14 into a lower chamber 12a and an upper chamber 12b.
As the cross-sectional area of the intermediate cylinder 16 is less than the first cylinder 14, an annular chamber 36 is formed between these two cylinders. Passages 38 are provided to maintain the chamber 36 in fluid communication with the interior of the intermediate cylinder 16.
In normal operation, there is no fluid communication between the lower chamber 12a and the upper chamber 12b. In order to extend the jack, fluid from reservoir 40 is pumped into the lower chamber 12a by pump 42, which pushes upwardly on piston 22. As the piston 22 begins to rise, the volume in chamber 36 begins to decrease, forcing fluid from the chamber 36 into the interior of the intermediate cylinder 16. The resultant pressure increase within the intermediate cylinder 16 pushes the inner plunger 32 outwardly relative to the intermediate cylinder 16 so as to maintain the overall volume in the upper chamber 12b substantially constant.
In telescoping jacks of this type, the intermediate cylinder 16 and the inner plunger 32 thus inherently move outwardly simultaneously. The jacks are designed so that the inner plunger reaches its outermost position, defined by stop 44, at the same time the intermediate cylinder reaches its outermost position, when the upper side 46 of the piston reaches the bearing housing 20 (alternatively, stops can be secured to the intermediate cylinder).
Initially, the upper chamber 12b is completely filled with hydraulic fluid. Over time, fluid tends to leak out through the seals 18 and 28, so that the upper chamber 12b is no longer completely filled with fluid. When this occurs, the intermediate cylinder 16 and inner plunger 32 are no longer able to extend their full range. It is thus necessary, from time-to-time, to resupply hydraulic fluid to the upper chamber 12b.
Thus, telescoping jacks are typically provided with a mechanism to transfer fluid from the lower chamber 12a to the upper chamber 12b. A simplified version of such a mechanism 50 is shown in FIG. 1. As shown, during normal operation of the elevator, flow of oil from the lower chamber 12a to the upper chamber 12b is blocked by piston 52, which is retained in sealed position in seat 54 by spring 56 and also by the pressure of the oil from chamber 12a.
If the car is lowered to its lowermost position, the stop 34 pushes the valve housing 58 downwardly, opening valve 52, 54 and allowing pressurized oil from the lower chamber 12a to flow into the upper chamber 12b. As soon as the car moves upwardly again a short distance, the spring 56 forces the valve 52, 54 closed again.
During normal elevator operation, when the elevator car is at the lowest floor, the jack is not at its lowest position, and thus fluid is not replenished into the upper chamber. Rather, replenishing oil into the upper chamber is normally done as part of elevator servicing. This operation, which is referred to as resynchronization, is well know and need not be described further here.
In an elevator having a single telescoping jack, loss of fluid in the upper chamber 12b means that an elevator car may not be able to reach the upper floor. In a twin post telescoping jack elevator, however, the problem can be more serious, because one jack can lose oil faster than the other and become out of synchronization with the other. Thus, the two jacks need to be re-synchronized, which is done by lowering the car so as to actuate the refilling mechanisms of both jacks.
If on dual post elevators one of the jacks becomes much more out of sync than the other, the out-of-sync jack may bottom out (i.e., reach the limit of its upward movement) while running up. This will cause one jack to stop moving while the other jack continues to extend, causing the car to rack.
Presently, this problem is dealt with by building the car sling strong enough to prevent the unbalance from racking it. This requires the car sling to be built with much more steel, which adds considerably to the cost of the elevator. It also results in increased power unit requirements to handle the extra weight.